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Chapter 11: Problem 69

For a given substance, the liquid crystalline phase tends to be more viscous than the liquid phase. Why?

Short Answer

Expert verified
The liquid crystalline phase tends to be more viscous than the liquid phase due to differences in molecular arrangement and interactions. In the liquid phase, molecules are randomly arranged and have weak intermolecular forces, allowing them to slide past each other easily, resulting in low viscosity. In the liquid crystalline phase, molecules are partially ordered and have stronger intermolecular forces due to elongated shape and alignment. This causes greater resistance to flow, making the liquid crystalline phase more viscous than the liquid phase.

Step by step solution

01

Understanding the liquid and liquid crystalline phases

Liquid phase is a state of matter where the substance has a definite volume but no definite shape, so it takes the shape of the container it is placed in. On the other hand, the liquid crystalline phase, also known as mesophase, is a state of matter that exhibits properties between those of crystalline solids and ordinary liquids. Liquid crystals can flow like a liquid but have long-range order like crystals. Their molecules tend to be elongated and partially orient themselves along a preferred direction.
02

Analyzing molecular arrangement

In a liquid phase, molecules are randomly arranged, and they have relatively weak interactions with each other due to their isotropic nature. They can easily move and slide past each other, which allows the liquid to flow freely. In the liquid crystalline phase, the molecules are partially ordered and tend to align in specific directions or planes. This results in a more organized structure than the liquid phase.
03

Understanding molecular interactions

In the liquid crystalline phase, due to the elongated shape of the molecules and their partial alignment, these molecules tend to have stronger intermolecular forces (such as van der Waals forces) between them than those in the liquid phase.
04

Understanding viscosity

Viscosity is a measure of a substance's resistance to flow. It is influenced by factors such as molecular arrangement, intermolecular forces, and temperature. When molecules are randomly arranged and have weak interactions (like in the liquid phase), they slide past each other easily, and their resistance to flow is low, resulting in low viscosity. When molecules are partially ordered and have stronger interactions (like in the liquid crystalline phase), they resist flowing and have difficulty sliding past each other. Their resistance to flow is higher, resulting in higher viscosity.
05

Comparing viscosity of liquid and liquid crystalline phases

Based on the analysis of molecular arrangement and intermolecular interactions, we can conclude that the liquid crystalline phase tends to have more organized molecular arrangements and stronger intermolecular forces than the liquid phase. This leads to a higher resistance to flow, resulting in a more viscous liquid crystalline phase as compared to the liquid phase.

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Most popular questions from this chapter

(a) When you exercise vigorously, you sweat. How does this help your body cool? (b) A flask of water is connected to a vacuum pump. A few moments after the pump is turned on, the water begins to boil. After a few minutes, the water begins to freeze. Explain why these processes occur.

Arrange substances \(\mathrm{CCl}_{4},\) Si, and Ar in order of increasing boiling point.

Ethylene glycol \(\left(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right)\) is the major component of antifreeze. It is a slightly viscous liquid, not very volatile at room temperature, with a boiling point of \(198^{\circ} \mathrm{C}\). Pentane \(\left(\mathrm{C}_{5} \mathrm{H}_{12}\right),\) which has about the same molecular weight, is a nonviscous liquid that is highly volatile at room temperature and whose boiling point is \(36.1^{\circ} \mathrm{C}\). Explain the differences in the physical properties of the two substances.

Based on what you have learned about intermolecular forces, would you say that matter is fundamentally attracted or repulsed by other matter?

The table shown here lists the molar heats of vaporization for several organic compounds. Use specific examples from this list to illustrate how the heat of vaporization varies with (a) molar mass, (b) molecular shape, (c) molecular polarity, (d) hydrogen-bonding interactions. Explain these comparisons in terms of the nature of the intermolecular forces at work. (You may find it helpful to draw out the structural formula for each compound.) $$ \begin{array}{ll} \text { Compound } & \begin{array}{l} \text { Heat of Vaporization } \\ \mathbf{( k J / m o l )} \end{array} \\ \hline \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3} & 19.0 \\ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} & 27.6 \\ \mathrm{CH}_{3} \mathrm{CHBrCH}_{3} & 31.8 \\ \mathrm{CH}_{3} \mathrm{COCH}_{3} & 32.0 \\ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br} & 33.6 \\ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} & 47.3 \end{array} $$
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